1. Field of the Invention
The invention relates to polymer electrolytes.
2. Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.”)
As our energy economy begins to rely on renewable, but more intermittent energy sources such as solar and wind, safe, high-energy density storage will be come increasingly important. Polymer electrolytes hold promise for the development of higher energy density lithium-ion batteries that can be adapted to a variety of applications. By eliminating the need for volatile, flammable, and toxic small-molecule electrolytes, these solvent-free solid-state batteries, in which a moderate to high molecular weight polymer is mixed with a lithium salt, also increase battery safety and introduce new degrees of design flexibility. The greatest motivation behind the design of solid polymer electrolytes is that highly-energetic metallic lithium anodes may be safely used in lieu of less-energetic intercalation compound anodes [1]. The dominant challenge to advancing these materials commercially has been the development of a polymer material that is able to dissolve and dissociate lithium salts and allow for lithium ion mobility while maintaining mechanical strength and electrode separation. To date, poly(ethylene oxide) (PEO) has been the most frequently and thoroughly studied polymer electrolyte due to its good solvating properties and conductivities greater than 10−4 S/cm above 70° C. However, below 65° C., PEO conductivity drops off dramatically as a result of crystallization, thus making PEO ill-suited for most battery applications that require operation at ambient temperatures. As a result, recent work has sought to modify PEO to eliminate crystallinity while retaining good solvating, conducting, and structural properties. Strategies employed include PEO oligomer crosslinking [2-7], synthesis of rubbery block copolymers [8,9] with or without incorporation of plasticizing agents [10], and most successfully, development of supramolecular architectures such as combs [11] and dendritic structures [12-14]. While these techniques have yielded increases in conductivity at lower temperatures, as yet they have been unable to meet the requisite conductivity of 10−3 S/cm deemed necessary for commercial viability. Moreover, continuing improvements have come at the cost of increasingly sophisticated synthetic schemes that might not be feasible on an industrial scale. Still yet other modifications have yielded systems that are increasingly similar to the small-molecule systems they are to replace [15].